Yet another braindump so I can free some cells... Acked-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx> --- virt/kvm/arm/vgic/vgic-v4.c | 67 +++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 67 insertions(+) diff --git a/virt/kvm/arm/vgic/vgic-v4.c b/virt/kvm/arm/vgic/vgic-v4.c index d10e18eabd3b..e367d65a0ebe 100644 --- a/virt/kvm/arm/vgic/vgic-v4.c +++ b/virt/kvm/arm/vgic/vgic-v4.c @@ -23,6 +23,73 @@ #include "vgic.h" +/* + * How KVM uses GICv4 (insert rude comments here): + * + * The vgic-v4 layer acts as a bridge between several entities: + * - The GICv4 ITS representation offered by the ITS driver + * - VFIO, which is in charge of the PCI endpoint + * - The virtual ITS, which is the only thing the guest sees + * + * The configuration of VLPIs is triggered by a callback from VFIO, + * instructing KVM that a PCI device has been configured to deliver + * MSIs to a vITS. + * + * kvm_vgic_v4_set_forwarding() is thus called with the routing entry, + * and this is used to find the corresponding vITS data structures + * (ITS instance, device, event and irq) using a process that is + * extremely similar to the injection of an MSI. + * + * At this stage, we can link the guest's view of an LPI (uniquely + * identified by the routing entry) and the host irq, using the GICv4 + * driver mapping operation. Should the mapping succeed, we've then + * successfully upgraded the guest's LPI to a VLPI. We can then start + * with updating GICv4's view of the property table and generating an + * INValidation in order to kickstart the delivery of this VLPI to the + * guest directly, without software intervention. Well, almost. + * + * When the PCI endpoint is deconfigured, this operation is reversed + * with VFIO calling kvm_vgic_v4_unset_forwarding(). + * + * Once the VLPI has been mapped, it needs to follow any change the + * guest performs on its LPI through the vITS. For that, a number of + * command handlers have hooks to communicate these changes to the HW: + * - Any invalidation triggers a call to its_prop_update_vlpi() + * - The INT command results in a irq_set_irqchip_state(), which + * generates an INT on the corresponding VLPI. + * - The CLEAR command results in a irq_set_irqchip_state(), which + * generates an CLEAR on the corresponding VLPI. + * - DISCARD translates into an unmap, similar to a call to + * kvm_vgic_v4_unset_forwarding(). + * - MOVI is translated by an update of the existing mapping, changing + * the target vcpu, resulting in a VMOVI being generated. + * - MOVALL is translated by a string of mapping updates (similar to + * the handling of MOVI). MOVALL is horrible. + * + * Note that a DISCARD/MAPTI sequence emitted from the guest without + * reprogramming the PCI endpoint after MAPTI does not result in a + * VLPI being mapped, as there is no callback from VFIO (the guest + * will get the interrupt via the normal SW injection). Fixing this is + * not trivial, and requires some horrible messing with the VFIO + * internals. Not fun. Don't do that. + * + * Then there is the scheduling. Each time a vcpu is about to run on a + * physical CPU, KVM must tell the corresponding redistributor about + * it. And if we've migrated our vcpu from one CPU to another, we must + * tell the ITS (so that the messages reach the right redistributor). + * This is done in two steps: first issue a irq_set_affinity() on the + * irq corresponding to the vcpu, then call its_schedule_vpe(). You + * must be in a non-preemptible context. On exit, another call to + * its_schedule_vpe() tells the redistributor that we're done with the + * vcpu. + * + * Finally, the doorbell handling: Each vcpu is allocated an interrupt + * which will fire each time a VLPI is made pending whilst the vcpu is + * not running. Each time the vcpu gets blocked, the doorbell + * interrupt gets enabled. When the vcpu is unblocked (for whatever + * reason), the doorbell interrupt is disabled. + */ + #define DB_IRQ_FLAGS (IRQ_NOAUTOEN | IRQ_DISABLE_UNLAZY | IRQ_NO_BALANCING) static irqreturn_t vgic_v4_doorbell_handler(int irq, void *info) -- 2.11.0
